This invention relates to a method of purifying aluminum and more particularly to the degassing of aluminum and aluminum alloys by passing a gaseous mixture of fluorine, hydrofluoric acid, carbon monoxide and carbon dioxide through the molten metal.
The aluminum metal in commercial use derives from two possible sources: it is either virgin aluminum derived from alumina, known as primary aluminum, or metal obtained by scrap recovery from many sources, known as secondary aluminum.
In both cases the metal has to be refined before it can be used for fabrication purposes. Oxides form dross and hydrogen dissolves in the metal, its solubility increasing with temperature. Unless removed, this dissolved gas causes flaws in the final cast products upon cooling. Another major problem is impurity elements which must either be completely removed from the molten metal or at least removed to a very low predetermined level.
It has been common practice in the aluminum and aluminum alloys industry to pass chlorine through molten metal in order to remove dissolved gasses and further to free the metal from porosity, oxide inclusions and other impurities. This process, known as fluxing or degassing, generally employs chlorine gas in full strength, i.e., 100 percent concentration. It has also been proposed to utilize nitrogen gas for degasification of aluminum. U.S. Pat. No. 3,149,960, discloses a fluxing gas containing a mixture of chlorine and carbon monoxide.
The process of contacting aluminum with a reactive chlorine-contained vapor is generally referred to as "chloridizing" aluminum. By this process, impurity metals and hydrogen are removed. Magnesium and sodium are converted to their chlorides and thus can be removed from the surface of the molten metal as a dross.
Generally, the treatment of aluminum is carried out by bubbling the chlorine gas into the molten metal while held in a melting or holding furnace or in a ladle. Chemical reaction between molten metal and chlorine insues, and chlorides are formed which rise to the surface of the metal as a dross, consisting of metallic chlorides, trapped particles of aluminum, and aluminum oxides. One disadvantage of this process is that the efficiency of the chlorine utilization is low. Further, during treatment, appreciable quantities of aluminum are lost from the molten bath as aluminum chloride. The excess chlorine which has to be used results in two problems. A part of the chlorine is lost as aluminum chloride. This hydrolyses on contact with atmospheric water to produce hydrochloric acid and a fume of extremely finely divided aluminum hydroxide or oxide. These two together constitute a formidable air pollution problem. Although the acid can be fairly effectively removed by a suitable water scrubbing system in the gas offtake, the alumina dust is so small, below 2 microns, that its removal is extremely difficult. Secondly, gaseous chlorine is lost from the melt and this can only be removed from the stack gasses by some form of reactive system. By the present invention not only are the problems and expense associated with the use of chlorine avoided, but a gaseous mixture currently regarded as waste in the production of aluminum by the electrolytic reduction of alumina is utilized. Presently the stack gas from the potlines of an aluminum electrolytic reduction plant not only are regarded as waste, but present air pollution control problems. The potline exhaust fumes must be scrubbed and cleaned by an elaborate system before these exhaust gases can be released into the atmosphere.
It is therefore an object of this invention to provide an improved method of degassing aluminum.
It is a further object of the present invention to provide a novel process of degassing aluminum using a gaseous mixture produced by the electrolytic reduction of alumina to aluminum.
Further objects and advantages of the present invention will become apparent from the following description.